Critical coronary artery obstruction reduces coronary blood flow, but the magnitude of coronary blood flow reduction producing myocardial dysfunction is unknown. We examined the quantitative effects of controlled coronary blood flow reductions upon myocardial function using sonomicrometry techniques. The ischemic area of myocardium becomes dilated and hypokinetic when coronary blood flow is reduced more than 25%. Sonomicrometry techniques detect changes in myocardial function with enhanced sensitivity over techniques which measure global myocardial function.